1. Field of the Invention
The present invention relates to a reflective type liquid crystal display device, and more particularly, to a reflective type hybrid alignment fringe field switching mode LCD (hereinafter, referred to as FFS-LCD).
2. Description of the Prior Art
A conventional reflective type LCD has generally employed a twisted nematic (TN) mode LCD in which liquid crystal composition having positive dielectric anisotropy is twist-aligned. The reflective type TN-LCD has an advantage of low power consumption and is used in small-sized LCD such as electronic table clocks and digital clocks. However, the reflective type TN-LCD has disadvantages of poor viewing angle properties and low contrast ratio.
In order to realize better viewing angle properties, higher reflectance, and higher aperture ratio, recent efforts are focused on research and development of a reflective type FFS-LCD. FIGS. 1 and 2 show outlines of the conventional FFS-LCD.
Referring to FIGS. 1 and 2, a lower substrate 11 and an upper substrate 12 are opposed with a predetermined distance. A liquid crystal layer 15 having a plurality of liquid crystal molecules 15a are interposed between the lower substrate 11 and the upper substrate 12. A counter electrode 14 and a pixel electrode 13 are arranged on the inner sides of the lower substrate 11, serving to form a fringe field for driving the liquid crystal molecules 15a. A color filter (not shown) is arranged on the inner side of the upper substrate 12. A first horizontal alignment layer 20 is interposed between the lower substrate 11 and the liquid crystal layer 15. And, a second horizontal alignment layer 19 is also interposed between the upper substrate 12 and the liquid crystal layer 15.
The first and the second horizontal alignment layers 20 and 19 have rubbing axes, respectively. The rubbing axis of the first horizontal alignment layer 20 forms an angle of 180xc2x0 (anti-parallel) with that of the second horizontal alignment layer 19. Also, the rubbing axis of the first horizontal alignment layer 20 forms a predetermined angle with a line obtained on the surface of the substrate by projecting a fringe filed formed between the counter electrode 14 and the pixel electrode 13. A polarizer 18 is attached on the outer side of the upper substrate 12, the polarizing axis thereof being consistent with the rubbing axis of the first horizontal alignment layer 20. A quarter wavelength plate 17 is arranged on the outer side of the lower substrate 11 so as to polarize the incident or reflective light. And, a reflector 16 is arranged on the outer side of the quarter wavelength plate 17 so as to reflect the light passed through the quarter wavelength plate 17. A fast or slow axis of the quarter wavelength plate 17 forms an angle of 45xc2x0 with the rubbing axis of the first horizontal alignment layer 20.
The operation of the conventional reflective type FFS-LCD will be explained in the following.
Referring to FIG. 1, when no voltage difference is generated between the counter electrode 14 and the pixel electrode 13, the liquid crystal molecules 15a are arranged, the major axes thereof being parallel with the rubbing axes of the horizontal alignment layers 20 and 19. Therefore, natural light becomes incident light moving toward the direction of the polarizing axis after passing through the polarizer 18. Afterwards, the incident light passes through the liquid crystal layer 15 wherein major axes of the liquid crystal molecules 15a are arranged to be parallel with rubbing axes of the horizontal alignment layers 20 and 19, and therefore, the moving direction of the incident light is not changed. Since the incident light forms an angle of 45xc2x0 with the fast or slow axis of the quarter wavelength plate 17, the incident light is changed into right or left circular polarized light through the quarter wavelength plate 17 after passing through the liquid crystal layer 15. The right circular polarized light is then reflected by the reflector 16 and changes left circularly polarized
A reflective light passes through the quarter wavelength plate 17 having the fast or slow axis forming an angle of 45xc2x0 with the moving direction thereof. Therefore, the moving direction of the reflective light is shifted to a direction perpendicular to the polarizing axis. Since the shifted direction of the reflective light is perpendicular to the major axes of the liquid crystal molecules 15a, the reflective light passes through the liquid crystal layer 15 without a change of the moving direction. Then, the reflective light is at right angles with the polarizing axis after passing through the liquid crystal layer 15, thereby not passing through the polarizer 18. As a result, a screen shows a dark state.
Referring to FIG. 2, when a fringe field (E) is formed between the counter electrode 14 and the pixel electrode 13, the liquid crystal molecules 15a are twisted in a shape of the fringe field. Therefore, optical axes of the liquid crystal molecules 15a form a predetermined angle with the polarizing axis. Passing through the polarizer 18, natural light becomes incident light moving toward the direction of the polarizing axis. Then, the incident light forms an angle of 45xc2x0 with the major axes of the liquid crystal molecules 15a arranged along the fringe field. Therefore, the incident light forms an angle of 45xc2x0 with the polarizing axis after passing through the liquid crystal layer 15. Since the incident light corresponds with the fast or slow axis of the quarter wavelength plate 17 after passing through the liquid crystal layer 15, the moving direction of the incident light is not changed when passing through the quarter wavelength plate 17. After passing through the quarter wavelength plate 17, the incident light is reflected by the reflector 16.
A reflective light passes through the quarter wavelength plate 17 without a change of the moving direction since the moving direction corresponds with the fast or slow axis of the quarter wavelength plate 17. The moving direction of the reflective light passing through the quarter wavelength plate 17 forms an angle of 45xc2x0 with the major axis of the liquid crystal molecules 15a of the liquid crystal layer 15, so that the moving direction of reflective light through the liquid crystal layer 15 corresponds with the polarizing axis. As a result, the screen shows a white state.
The conventional reflective type FFS-LCD has, however, several problems.
The conventional reflective type FFS-LCD additionally uses optical members such as the quarter wavelength plate on the outer side of the substrate in order to improve contrast without employing back light as a light source. This may cause an increase of manufacturing cost. Moreover, the transmittance and the reflectance of the LCD are lowered since the quarter wavelength plate does not generally convert the linear polarized incident lights into circular polarized light across all the wavelength or visa versa.
In order to solve the above-mentioned problems, especially in cost point of view, other method has been proposed that the liquid crystal layer substitutes for the quarter wavelength plate by making xcex(2n+1)/4 of the phase retardation (dxcex94n) of the liquid crystal layer.
However, this method as well has several drawbacks, which will be explained in the following.
Although the phase retardation of the liquid crystal layer is controlled to xcex(2n+1)/4, this phase retardation may serve to realize a dark state in only a specific range of wavelengths considering that the phase retardation is a function of light wavelength (xcex). Therefore, complete contrast is not obtained in whole ranges of wavelengths.
Furthermore, in order to determine the initial arrangement direction of the liquid crystal molecules, it is required to perform a rubbing process for forming rubbing axes on alignment layers of the upper and lower substrates. As well known in the art, the rubbing process may cause faults in process and surface damage of the alignment layer. Due to such problems, liquid crystal molecules may be misaligned in the dark state, and further, light may leak out. Unfortunately, this may result in poor screen characteristics of the FFS-LCD.
It is therefore an object of the present invention to provide a reflective type FFS-LCD device with improved contrast.
In order to achieve the above and other objects, the present invention provides a liquid crystal display device of a reflective type fringe field switching mode, comprising: a lower and an upper substrates arranged with a distance and having a unit pixel defined therein; a liquid crystal layer having a plurality of liquid crystal molecules and interposed between the lower and the upper substrates; a counter electrode formed on an inner surface of the lower substrate in the unit pixel; a pixel electrode formed above the counter electrode, wherein the pixel electrode and the counter electrode generate a fringe field for driving the liquid crystal molecules in the unit pixel; a horizontal alignment layer interposed between the lower substrate and the liquid crystal layer and having a rubbing axis; a vertical alignment layer interposed between the upper substrate and the liquid crystal layer; and a polarizer disposed on an outer surface of the upper substrate and having a polarizing axis.
The polarizing axis of the polarizer may correspond with or forms an angle of 45xc2x0 with the rubbing axis of the horizontal alignment layer. When the polarizing axis corresponds with the rubbing axis, the rubbing axis may form an angle of 45 to 90xc2x0 with a projected line of the fringe field on the substrates if the liquid crystal molecules have positive dielectric anisotropy. Also, the rubbing axis may form an angle of 0 to 45xc2x0 with the projected line if the liquid crystal molecules have negative dielectric anisotropy. Furthermore, in applying the fringe field, an effective phase retardation of the liquid crystal layer dxcex94n, where d is a distance between the lower and the upper substrates and xcex94n is a refractive anisotropy of the liquid crystal molecules, is xcex(2n+1)/4, where n is an integer.
Alternatively, when the polarizing axis of the polarizer forms an angle of 45xc2x0 with the rubbing axis, the rubbing axis may form an angle of 45 to 90xc2x0 with a projected line of the fringe field on the substrates if the liquid crystal molecules have positive dielectric anisotropy. And, if the liquid crystal molecules have negative dielectric anisotropy, the rubbing axis may form an angle of 0 to 45xc2x0 with with the projected line. Furthermore, before applying the fringe field, an effective phase retardation of the liquid crystal layer dxcex94n, where d is a distance between the lower and the upper substrates and xcex94n is a refractive anisotropy of the liquid crystal molecules, is xcex(2n+1)/4, where n is an integer.
Preferably, a refractive anisotropy xcex94n of the liquid crystal molecules may be at the range of 0.04 to 0.2, and a distance between the lower and the upper substrates may be 2 to 10 xcexcm. In addition, the liquid crystal layer may include dopants for serving to make the liquid crystal molecules easily twisted in applying field. Also, the liquid crystal display device may further comprise a reflector disposed near the lower substrate so as to reflect an incident light from the upper substrate. The counter-electrode is preferably made of one selected from a group consisting of aluminum, gold and silver.